1. Field of the Invention
This invention is concerned with an improved catalytic process for the demetalation and desulfurization of petroleum oils, preferably those residual fractions with undesirably high metals and/or sulfur contents. More particularly, the invention utilizes a demetalation-desulfurization catalyst characterized by novel specifications including pore size distribution, said catalyst comprising a Group VI A metal and an iron group metal composited with a Delta and/or Theta phase alumina, said catalyst having a specific pore size distribution, and other specific characteristics described hereinbelow.
2. Description of the Prior Art
Residual petroleum oil fractions produced by atmospheric or vacuum distillation of crude petroleum are characterized by relatively high metals and sulfur content. This comes about because practically all of the metals present in the original crude remain in the residual fraction, and a disproportionate amount of sulfur in the original crude oil also remains in that fraction. Principal metal contaminates are nickel and vanadium, with iron and small amounts of copper also sometimes present. Additionally, trace amounts of zinc and sodium are found in some feedstocks. The high metals content of the residual fractions generally preclude their effective use as charge stocks for subsequent catalyst processing such as catalytic cracking and hydrocracking. This is so because the metal contaminants deposit on the special catalysts for these processes and cause the premature aging of the catalyst and/or formation of inordinate amounts of coke, dry gas and hydrogen.
It is current practice to upgrade certain residual fractions by a pyrolytic operation known as coking. In this operation the residuum is destructively distilled to produce distillates of low metals content and leave behind a solid coke fraction that contains most of the metals. Coking is typically carried out in a reactor or drum operated at about 800.degree. to 1100.degree. F. temperature and a pressure of one to ten atmospheres. The economic value of of the coke by-product is determined by its quality, especially its sulfur and metals content. Excessively high levels of these contaminants make the coke useful only as low-valued fuel. In contrast, cokes of low metals content, for example up to about 100 ppm (parts-per-million by weight) of nickel and vanadium, and containing less than about 2 weight percent sulfur may be used in high valued metallurgical, electrical and mechanical applications.
Certain residual fractions are currently subjected to visbreaking, which is a heat treatment of milder conditions than used in coking, in order to reduce their viscosity and make them more suitable as fuels. Again, excessive sulfur content sometimes limits the value of the product.
Residual fractions are sometimes used directly as fuels. For this use, a high sulfur content in many cases is unacceptable for ecological reasons.
At present, catalytic cracking is generally done utilizing hydrocarbon chargestocks lighter than residual fractions which generally have an API gravity less than 20. Typical cracking chargestocks are coker and/or crude unit gas oils, vacuum tower overhead, etc., the feedstock having an API gravity from about 15 to about 45. Since those cracking chargestocks are distillates, they do not contain significant proportions of the large molecules in which the metals are concentrated. Such cracking is commonly carried out in a reactor operated at a temperature of about 800.degree. to 1500.degree. F., a pressure of about 1 to 5 atmospheres, and a space velocity of about 1 to 1000 WHSV.
The amount of metals present in a given hydrocarbon stream is often expressed as a chargestock's "metals factor." This factor is equal to the sum of the metals concentrations, in parts per million, of iron and vanadium plus ten times the concentration of nickel and copper in parts per million, and is expressed in equation form as follows: EQU F.sub.m =Fe+V+10(Ni+Cu)
Conventionally, a chargestock having a metals factor of 2.5 or less is considered particularly suitable for catalytic cracking. Nonetheless, streams with a metals factor of 2.5 to 25, or even 2.5 to 50, may be used to blend with or as all of the feestock to a catalytic cracker, since chargestocks with metals factors greater than 2.5 in some circumstances may be used to advantage, for instance with the newer fluid cracking techniques.
In any case, the residual fractions of typical crudes will require treatment to reduce the metals factor. As an example, a typical Kuwait crude, considered of average metals content, has a metals factor of about 75 to about 100. As almost all of the metals are combined with the residual fraction of a crude stock, it is clear that at least about 80% of the metals and preferably at least 90% needs to be removed to produce fractions (having a metals factor of about 2.5 to 50) suitable for cracking chargestocks.
Metals and sulfur contaminants would present similar problems with regard to hydrocracking operations which are typically carried out on chargestocks even lighter than those charged to a cracking unit. Typical hydrocracking reactor conditions consist of a temperature of 400.degree. to 1000.degree. F. and a pressure of 100 to 3500 psig.
It is evident that there is considerable need for an efficient method to reduce the metals and/or sulfur content of petroleum oils, and particularly of residual fractions of these oils. While the technology to accomplish this for distillate fractions has been advanced considerably, attempts to apply this technology to residual fractions generally fail due to very rapid deactivation of the catalyst, presumably by metals contaminants.
Hydrotreatment catalysts having specified pore distributions have been proposed to overcome disadvantages encountered when using conventional prior art catalysts for the hydrotreatment of petroleum residua or other metals and sulfur-containing, heavy hydrocarbons.
Rosinski (U.S. Pat. No.4,082,695) discloses a hydrodemetalation-desulfurization class of catalysts comprising a hydrogenating component (e.g., cobalt and molybdenum) composited with a particular refractory base comprising theta or delta phase alumina. The composite catalyst of Rosinski has a surface area of about 40-150 square meters per gram (m.sup.2 /g) and has the following pore size distribution: not less than 60% of the total pore volume have a diameter within the range of about 100-200 Angstroms (A), not less than about 5% of the total pore volume are greater than 500 A in diameter. The preferred catalyst has a surface area of 110 m.sup.2 /g or less and not less than 5% of the total pore volume are less than about 40 A in diameter. The efficiency of the catalyst is principally a result of the high concentration or pores within the 100-200 A range although the largest pores (greater than about 500 A) are said to be required for conversion of exceptionally large heteroatomic molecules and the smallest pores (less than about 40 A) are though to enhance sulfur removal generally. The distinct pore size distribution of the catalyst is believed to be due, at least in part, to the calcination of the alumina catalyst base during preparation to produce a specific alumina comprising theta or delta phase alumina.
U.S. Pat. Nos. 3,876,523; 4,016,067; and 4,054,508 disclose processes for the demetalation and desulfurization of residua which employ the Rosinski catalyst. The '523 patent discloses and claims this use of the catalyst generally. The '067 patent discloses a dual catalyst system wherein the Rosinski catalyst in the first "demetalation" catalyst and a high surface area, smaller pore catalyst is the second, "desulfurization" catalyst. The '508 patent discloses a three-zone, dual catalyst process which is analogous to the '067 process except that there is an additional, third zone containing a relatively smaller bed of the first zone catalyst disclosed by Rosinski.
U.S. Pat. Nos. 4,048,060 and 4,069,139 disclose an alumina-containing hydrotreating catalyst having a mean pore radius of about 70 to 95 A, a total pore volume between 0.45 and 1.50 milliliters per gram (ml/g), a total surface area between 130 and 500 m.sup.2 /g, and the following pore size distribution: less than 0.05 ml of pore volume/g have radii greater than 100 A, at least 0.40 ml of pore volume/g have radii in the range of the mean pore radius .+-.10 A, at least 75% of the total pore volume have radii in the range of the mean pore radius .+-.10 A, and less than 0.05 ml of pore volume/g have radii below 60 A. The method of preparing this hydrotreating catalyst and its alumina support are "conventional." U.S. Pat. No. 4,048,060 at col. 6, lines 30-36 and U.S. Pat. No. 4,069,139 at col. 4, lines 55-60. "Conventional" alumina supports comprise gamma alumina and catalysts prepared from such supports do not have the advantageous properties of catalysts such as those of Rosinski, supra, to which the catalysts of the present invention are related.
Other less relevant patents in this general area are: Anderson (U.S. Pat. No. 2,890,162), Erickson (U.S. Pat. No. 3,242,101), Bertolacini (U.S. Pat. No. 3,393,148), Cornelius (U.S. Pat. No. 3,669,904), Roseline (U.S. Pat. No. 3,684,688), Bertolacini (U.S. Pat. No. 3,714,032), Christman (U.S. Pat. No. 3,730,879), Wilson (U.S. Pat. No. 3,898,155), Oleck (U.S. Pat. No. 3,931,052), Hamner (U.S. Pat. No. 4,014,821), and Oleck (U.S. Pat. No. 4,089,774).